The present invention relates to a system and method for detecting a predefined signal or tone where such signal has a low signal to noise ratio.
In a cordless telephone set having a base-station and a handset, the base-station and the handset periodically exchange control signals. For example, a control signal from the handset might instruct the base-station to release a telephone line. These control signals are digitally encoded messages contained in brief bursts lasting a few hundred milliseconds.
Duplex radio links already exist between the base-station and the handset to carry the telephone user""s conversation signals. It is desirable to use this existing link to carry the control signals as well. As a result, the control signals use the same audio bandwidth as the conversation signals.
However, merely superimposing the control signals onto the conversation signals would be extremely annoying and disturbing to the user as such control signals would be audible to the user. Moreover, the user""s conversation signals might corrupt the control signals by adding noise. For this reason, it is desirable that the audio channel between the base-station and the handset be time-division multiplexed to independently carry either conversation or control signals.
In a default xe2x80x9cvoice modexe2x80x9d, the audio channel carries only conversation signals. In an occasional xe2x80x9cdata modexe2x80x9d, the audio channel carries only control signals. To effect this kind of a esynchronous multiplexing, a handshaking signal is used.
Before either the base-station or the handset transmits a control signal, it is desirable that it indicate its intention to switch from xe2x80x9cvoice modexe2x80x9d to xe2x80x9cdata modexe2x80x9d so that both devices can mute their microphones and loudspeakers. This intention is normally indicated by inclusion of a pilot signal in a transmission from the base or handset.
The pilot signal is carried over the established radio link without disturbing the user. It is an audio tone having a predetermined frequency proximate to but outside of the telephone set""s voice-band. This prevents the user from hearing any unnatural sounds or breaks in conversation.
It is important that pilot tone detection be robust. Detection must occur over a range at least as great as the telephone set voice-channel and should occur even at low signal to noise ratios. A receiver that incorrectly detects a non-existent pilot tone will unnecessarily mute its audio path. A receiver that fails to detect a real pilot tone will expose its user to an unpleasant databurst. This creates a requirement that the pilot tone path be more robust than the audio channel.
The present invention addresses the problem of pilot tone detection at low signal to noise ratios.
In accordance with one aspect of the invention, there is provided an apparatus for detecting the presence of a signal component in an arbitrary signal, where the signal component has a desired frequency. The apparatus includes an error signal generator and a detector. The error signal generator has an input for receiving a reference signal having a reference frequency, a second input for receiving the arbitrary signal having the signal component and an output for generating a difference signal dependent upon a difference in frequency between the reference frequency and the desired frequency. The detector has an input for receiving the difference signal and an output for producing a detector signal when the magnitude of the difference signal is greater than a threshold magnitude. The detector signal indicates the presence of the signal component in the arbitrary signal.
Preferably, the difference signal has a magnitude dependent upon the difference in frequency between the reference frequency and the desired frequency.
It is desirable that the error signal generator includes a duty cycle signal generator for generating a duty cycle signal having a duty cycle responsive to the difference between the reference frequency and the desired frequency.
Preferably, the error signal generator includes a phase locked loop.
It is desirable that the phase locked loop has an input for receiving the arbitrary signal and the reference signal, the arbitrary signal and the reference signal having respective leading edges, the phase locked loop producing the duty cycle signal in response to a time difference between the respective leading edges.
Preferably, the error signal generator includes a lowpass filter having an input for receiving the duty cycle signal and an output for producing the difference signal.
It is desirable that the apparatus includes a threshold signal generator for generating a threshold signal, the detector having an input for receiving the threshold signal.
Preferably, the threshold signal generator has an input for receiving a supply voltage signal representative of a supply voltage from a power supply supplying power to the error signal generator and the detector, and an output for generating the threshold signal in response to the supply voltage signal.
It is desirable that the threshold signal generator includes a voltage divider.
Preferably, the apparatus includes a bandpass filter connected to the arbitrary signal input, for bandpass filtering the arbitrary signal.
It is desirable that the bandpass filter includes a lowpass filter and a highpass filter.
In accordance with another aspect of the invention, there is provided a method for detecting the presence of a signal component in an arbitrary signal where the signal component has a desired frequency. The method includes the steps of:
a) receiving a reference signal having a reference frequency;
b) receiving the arbitrary signal having the signal component;
c) generating a difference signal dependent upon a difference in frequency between the reference frequency and the desired frequency; and
c) producing a detector signal when the magnitude of the difference signal is greater than a threshold magnitude, the detector signal indicating the presence of the signal component in the arbitrary signal.
In accordance with another aspect of the invention, there is provided a signal tone detector apparatus for detecting a signal tone component having a desired frequency in a radio frequency signal in a cordless telephone having a base-station and a handset. The apparatus includes a radio frequency demodulator, an error signal generator and a detector. The radio frequency demodulator demodulates a baseband signal from the radio frequency signal transmitted between the base-station and the handset. The error signal generator has a first input for receiving a reference signal having a reference frequency, a second input for receiving the baseband signal having the signal component and an output for generating a difference signal dependent upon a difference in frequency between the reference frequency and the desired frequency. The detector has an input for receiving the difference signal and an output for producing a detector signal when the magnitude of the difference signal is greater than a threshold magnitude, the detector signal indicating the presence of the signal component in the arbitrary signal.